Many electronic devices include projected capacitive touch screens, commonly referred to as PCT or PCAP screens, such as smart phones, tablets, e-readers, etc. A typical PCAP screen is made up of a matrix of rows and columns of conductive material, layered on sheets of glass. This can be done either by etching a single conductive layer to form a grid pattern of electrodes, or by etching two separate, perpendicular layers of conductive material with parallel linear electrodes or “tracks” to form a grid. The electrodes of the single conductive layer in the former approach or the location where the linear electrodes of each layer overlap in the latter approach form the sensor nodes of the touch screen. Voltage applied to this grid creates a electrostatic field, which can be measured. When a object with enough additional charge-holding capability, such as a fingertip, comes into contact with a PCAP panel, it alters the local electrostatic field at that point and thus produces a measurable change in the charge of the electrodes at or near the point of contact. The capacitance can be changed and measured at every individual point on the grid (intersection). Accordingly, the location of the center of the touch can be determined at a resolution finer than the spacing between the electrodes by interpolation between affected adjacent electrodes. Therefore, this system is able to track touches by a fingertip or other object that produces a measurable change at a plurality of adjacent electrodes at a resolution much better than the spacing of those electrodes; for example, a touch screen with electrodes spaced 5 millimeters apart may be able to interpolate the position of the center of a touch that affects multiple electrodes with an accuracy better than 0.5 millimeters. However, if the object produces a measurable change at only one electrode, interpolation is not possible and the position of the touch can only be measured at the resolution of the electrode spacing.
Although a PCAP screen can typically sense some types of passive stylus, most PCAP screens currently on the market are designed to interact with a finger. Accordingly, a PCAP screen may require that the area of contact with the screen have a minimum centroid area of about ¼ inch in diameter in order to detect a touch. However, a finger does not produce a very good user experience when trying to emulate the actions of a writing utensil such as a pen or pencil due to the physical size of the point of contact required for detection.
FIG. 1 illustrates a portion of a conventional PCAP touch screen, showing an array of sensor nodes N defined at the intersection of orthogonal sensor tracks T. A contact area formed by a finger in contact with the touch screen is indicated by the dashed line A. Each sensor node within the contact area A is activated, that is, has a measurable change in its stored charge or its capacitance, as indicated by an enlarged dot at each activated node Na. Signals from the array of sensors are communicated to a controller, which calculates a centroid area and/or center of area A based on the activated nodes Na. The controller may require a minimum contact area, diameter, or other dimension in order to identify the detected contact area A as a user input.
FIGS. 2A-2C illustrate side views of example objects contacting a conventional PCAP touch screen having a controller that require a minimum contact area, diameter, or other dimension, indicated as Amin, to detect a user input. In some designs, the minimum contact area may be defined by a minimum number of activated nodes Na. FIG. 2A illustrates a finger in contact with the screen, wherein the area of contact is greater than the minimum contact dimension Amin, and thus the controller detects a user input. Similarly, FIG. 2B illustrates a stylus SLT having a large tip LT in contact with the screen, wherein the area of contact is greater than the minimum contact dimension Amin, and thus the controller detects a user input. However, FIG. 2C illustrates a stylus SFP having a fine-point tip FT in contact with the screen, wherein the area of contact is less than the minimum contact dimension Amin, and thus the controller does not detect a user input at multiple electrodes (or even at one electrode if the fine-point tip is sufficiently far from the closest electrode) and therefore the controller can not accurately determine the location of the touch.
Thus, passive styli currently used for such screens achieve this minimum centroid area using a relatively large tip, e.g., in the form of a relatively large rubber or foam tip. However, this large tip obscures the user's view of the point of contact with the screen. Alternatively, some PCAP screens provide an increased sensor density (e.g., increased line or track density) in order to detect a fine-point stylus. However, increasing the sensor density typically increases the costs for both the sensor array and the associated controller. These disadvantages are even more pronounced in devices with larger touch screens, e.g., tablets and e-readers.